1. Field of the Invention
The present invention relates to a screen printing method and apparatus which are suitable for formation of a fine pattern film of phosphor material, etc. in a color plasma display panel.
2. Description of the Related Art
A flat display apparatus, such as a color plasma display panel, is required to print fine pattern films, and usually relies upon phosphor material, separation walls and electrodes to generate accurate and high quality displays.
FIG. 1 shows a screen mask for explaining the screen printing method and printing apparatus of the related art. FIG. 1(a) is a perspective view, and FIG. 1(b) is a cross-sectional view taken along the line a-a' of FIG. 1(a).
As shown in FIGS. 1(a) and (b) screen printing is performed using a screen mask 31 in which a screen 33 having a plurality of apertures 33a formed therein in a stripe pattern corresponding to a print pattern is supported by a metal frame 32 with predetermined tension. This screen mask 31 is arranged to maintain a predetermined space for a printing object.
For example, in the case of printing a phosphor material of a color plasma display panel (hereinafter referred to as PDP), a glass substrate, having formed thereon a plurality of address electrodes, a dielectric material layer covering said address electrodes and a plurality of separation walls or stripes is positioned on the lower side of the screen mask 31 and is thereafter aligned with the apertures 33a of the screen 33 of the screen mask 31. The positioning is made so that narrow grooves or elongated cavities formed between the separation walls on the glass substrate oppose the aperture 33a stripes.
After such positioning, the phosphor material paste of the predetermined color, for example, red is applied onto the screen 33 of the screen mask 31 to fill the grooves between the predetermined separation walls on the glass substrate via the apertures 33a by sliding a squeegee on the screen 33.
After application of the red phosphor material paste, the grooves between the separation walls corresponding to green and blue are also filled with blue and green phosphor material pastes, respectively, in the same manner using screen masks with different aperture positions. Thereby, the printing of the phosphor materials can be completed.
With continued efforts to improve the display resolution and overall quality, a pitch of the grooves between the separation walls by which the phosphor materials are printed tends to become finer and thereby the screen mask 31 used for printing of the phosphor materials has apertures 33a of finer pitch. Moreover, the display screen has recently been enlarged and thereby the number of apertures has also increased.
However, it has been difficult in the known screen printing apparatus and method to maintain highly accurate printing using such a finer pitch pattern.
Namely, in the printing method using the screen mask as explained above, printing accuracy can be lowered by size error due to delicate changes in the printing conditions such as squeegee pressure and viscosity of the phosphor material paste, or by a change in the printing position due to expansion and compression of the screen itself depending on the number of times of use or the passage of time.
Lowered printing accuracy causes, in the case of the fine pattern, a shortage in the amount of phosphor material paste to be supplied to the grooves between the separation walls and, moreover, excessively lowered printing accuracy will generate serious defects such as printing to an unwanted area, for example, of the red phosphor material, when the green phosphor material was intended.
A size error depending on the printing condition may be controlled to a certain degree by predicting the printing condition at the time of mask design. Therefore, generation of serious defects as explained above can be prevented, but there is a limit to the number of times of use of the mask, and replacement of the mask is required when an error is generated.
In addition, since a longer time is required for the design of the screen mask, even if a designer is a skilled person, change of the mask pattern because of generation of size error or change of specifications cannot be done easily and, moreover, it is difficult to take an immediate measure.